1. Field of the Invention
The present invention relates to an OFDM receiver for receiving and demodulating an OFDM signal, and in particular, relates to a method for adjusting a filter for interpolating a pilot signal in an OFDM signal of digital terrestrial broadcasts in a frequency domain.
2. Description of the Related Art
As a system transmitting a digital signal, an Orthogonal Frequency Division Multiplexing (OFDM) has been proposed in recent years. In the OFDM system, data is transmitted employing a plurality of carriers orthogonal to each other in the frequency domain. For that reason, an OFDM transmitter modulates a transmission signal by utilizing Inverse Fast Fourier Transformation (IFFT), and an OFDM receiver demodulates the transmission signal by using Fast Fourier Transformation (FFT) Since the OFDM system has high frequency efficiency, application to the digital terrestrial broadcasts has been widely explored. It should be noted that OFDM has already been employed in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), which is a standard of the digital terrestrial broadcasts in Japan.
FIG. 1 is a diagram showing a configuration of a common OFDM receiver. In an OFDM receiver 100 shown in FIG. 1, an OFDM signal received via an antenna is fed to a tuner 101. The tuner 101 selects a signal of a desired channel from the received signal, and outputs the selected signal after converting into a signal in an intermediate frequency (IF) band. An A/D converter 102 converts the output signal of the tuner 101 into a digital signal. The digital signal is converted into a complex baseband signal by an orthogonal demodulator 103. The complex baseband signal, which is a time-domain signal, is converted into a frequency-domain signal by an FFT circuit 104. As a result, a plurality of signals transmitted by corresponding carriers, each having different frequencies, are obtained.
In addition to the data signal, a scattered pilot (SP) signal, an auxiliary channel (AC) signal, and a transmission and multiplexing configuration control (TMCC) signal etc. are transmitted in the digital terrestrial broadcasts. AC and TMCC are demodulated by a DQPSK demodulator apparatus, not shown in the drawing, and TMCC information including transmission parameters is extracted.
A data carrier transmitting the data signal and an SP carrier transmitting the scattered pilot signal (hereinafter referred to as the SP signal) are input to an equalization processing unit 105. The SP signal is a known signal, having the transmission phase and the transmission power determined in advance, and is used for synchronous detection and transmission path estimation (channel estimation). The equalization processing unit 105 performs interpolation processing of the SP signal. The equalization processing unit 105 equalizes the data signal using the result of the interpolation processing, and outputs the equalized data signal as demodulated data. In this description, the term “equalization” includes processing for correcting phase rotation occurred on the transmission path. The demodulated data is converted into binary data with one bit or a plurality of bits by de-mapping processing, and is output in Transform Stream (TS) format after correction processing by an error correction circuit 106.
FIG. 2 is a diagram showing the arrangement of the SP signal. The SP signal is inserted every 12 carriers in the frequency domain. Each carrier is provided at 1 kHz intervals in the mode 3 of the digital terrestrial broadcasts, for example. The SP signal is inserted every 4 symbols in the time domain. 1 symbol time is, for example, 1.008 ms. In the example shown in FIG. 2, the SP signals are transmitted using carriers C1, C13, . . . in a time slot for transmitting the nth symbol, and the SP signals are transmitted using carriers C4, C16, . . . in a time slot for transmitting the n+1th symbol.
FIG. 3 is a diagram showing a configuration of the equalization processing unit 105. The equalization processing unit 105 comprises an SP interpolation unit 110 and a complex division unit 120. The SP interpolation unit 110 comprises a symbol interpolation unit 111 and a carrier interpolation unit 112. The symbol interpolation unit 111 performs interpolation processing of each carrier transmitting the SP signal in the time domain. In the example of FIG. 2, for example, with respect to the carrier C1, signals of time slots N+1, N+2, and N+3 are estimated based on the signal of the time slot N and the signal of the time slot N+4. The same interpolation processing is performed for other carriers (C4, C7, C10, . . . ), on which the SP signals are arranged. As a result, in each of the carriers C1, C4, C7, C10, . . . , to which the SP signals are inserted, information of all symbols can be obtained.
The carrier interpolation unit 112 is, for example, a digital filter such as a FIR filter or an IIR filter, and performs interpolation processing in the frequency domain using the interpolation result of the symbol interpolation unit 111. In other words, in each time slot, by employing the signals of the carriers C1, C4, C7, . . . , signals of the carriers C2, C3, C5, C6, C8, C9, . . . are estimated. As a result, reception information of the SP signal in all carriers can be obtained. At that time, since the transmission phase and the transmission power of the SP signal are determined in advance, based on the reception information of the SP signal, the transmission path characteristic information (phase information etc.) of the SP signal can be obtained. That is, the SP interpolation unit 110 generates transmission path characteristic information for all carriers.
The complex division unit 120 corrects data signal by complex division calculation using the transmission path characteristic information obtained as described above. As a result, the data signals are equalized so as to remove the influence of the transmission path.
It should be noted that the OFDM receiver is described in Patent Documents 1-4, for example. The Patent Document 1 describes a technology for switching the coefficients of the filter comprising the equalization processing unit in accordance with the conditions of the transmission path. The Patent Document 2 describes a technology for adjusting a window position of FFT in accordance with the delay waves. The Patent Document 3 describes a technology for estimating the transmission path by performing Fourier transformation of the signal after removing interfering signal components based on the delay profile. The Patent Document 4 describes a technology for controlling the coefficient of the filter constituting the equalization processing unit based on the error rate of the received data.
[Patent Document 1]
Japanese Patent Application Publication No. 2002-64464
[Patent Document 2]
Japanese Patent Application Publication No. 2001-292125
[Patent Document 3]
Japanese Patent Application Publication No. 2004-266814
[Patent Document 4]
Japanese Patent Application Publication No. 2002-26861
In OFDM, in order to enhance the tolerance to multipath (reflected waves of a radio wave transmitted from a base station are generated, and the radio waves propagating the same signal arrive at one terminal via a plurality of paths in TV/Radio broadcasts or mobile telephone system), a guard interval is inserted into each interval of the symbols. The guard interval is obtained by adding the signal in the end portion of a symbol immediately before the symbol. At that time, the guard interval period is generally determined so that interference between symbols (a condition such that a signal of a symbol and a signal of the subsequent symbol are received at the same time) does not occur in an assumed multipath environment. In the case of the digital terrestrial broadcasts, the guard interval is ⅛ symbol time (i.e. 126 μs). Consequently, in general, the receiver can demodulate a signal to regenerate transmission data even under multipath environment in the OFDM system.
The interference between symbols occurs when the delay time of the multipath is larger than the guard interval, and reception quality is deteriorated. At that time, the interference between symbols rarely occurs since the guard interval period is generally determined to be larger than the assumed delay time. However, in some communication environments, delay time of multipath may be larger than the guard interval. For example, in a situation where radio waves propagating the same signal are received from two base stations, the time lag between a radio wave from one base station and a radio wave from the other base station may exceed the guard interval. In such a case, the interference between symbols practically occurs, and the reception quality is deteriorated. Note that the environment where the delay time of multipath exceeds the guard interval had been hardly envisioned up to now.